V型環狀衝擊燃燒器之丙烷預混火焰特性研究

Abstract

本文測試自行開發之V型環狀衝擊燃燒器並分析火焰特性及流場結構、驗證新設計優良燃燒性能，分為兩部分進行探討，首先以實驗分析方法探討環狀平面及具V型環狀衝擊結構下丙烷預混火焰之特性，分別改變雷諾數 (Re = 300 ~ 800) 及當量比 (ϕ = 0.7 ~ 3.0)，透過量測火焰溫度場及使用粒子影像測速法 (PIV)，觀察及計算流場之變化。結果顯示在當量比變化下，可觀察到四種火焰型態在火焰結構以及效能上皆有所差異，其中藍焰 (ϕ = 1.0) 的火焰型態，穩定且受周圍流場影響小，無明顯擾動，溫度性能最佳。與環狀平面燃燒器比較，V型環狀衝擊結構可擴展穩定操作區間，且由溫度分布圖可觀察到V型環狀衝擊燃燒器於出口範圍預熱效應較好，速度場及渦度場量值因衝擊結構及剪應力增強為環狀平面燃燒器之兩倍，由流場可視化也觀察強勁之渦漩結構產生且捲進外圍空氣，使得燃料與助燃劑混合效果增強，再透過分析層流火焰週期性變化，可觀察渦漩結構產生，以及隨其發生之頸縮現象。各方面實驗結果可證明，V型環狀衝擊結構使得流場內聚、火焰間交互作用提升，預熱效應明顯，提升整體燃燒效能。 第二部分則以第一部分實驗結果為基礎，以增強火焰間交互作用為目標，分別調整整體尺寸大小以及衝擊角度 (30度、45度及60度) 進行探討，結果顯示內焰間彼此間隔明顯縮小，當衝擊角度為60度時，溫度表現最好；而當衝擊角度為30度時，其速度及渦度量值皆為最高，渦度場之量值同時受到速度及衝擊角度影響而在不同角度下有所差異，端看使用面向決定何種衝擊角度為佳。藉由本文分析V型環狀衝擊結構對於燃燒流場之影響，以期能建構此燃燒衝擊流場之應用基礎，未來開發同時兼顧燃燒效能及降低環境汙染之新型燃燒載具。

In this work, the innovative burner with v-shaped circular impinging structure was proposed. An experimental study was conducted to investigate the influence of burner structure on the feature of premixed propane flame. The design applied in the burner may reduce the consumption of fuel and improve the performance simultaneously. There are two main sections in this thesis. Firstly, the superior performance was verified through the comparison between v-shaped circular impinging burner and circular planar burner. The v-shaped circular impinging structure was beneficial to the numerous characteristics of combustion. The experimental results exhibited that blue flame (ϕ = 1.0) was the most stable flame type in the four types identified and was less influenced by surrounding flow filed from the high-resolution pictures took. This mechanism of impinging flame extended the stable operating region and effectively reduced the limit of blow-out flame and lift-off flame happening because of the preheated effect. Based on distribution of the temperature field, the high temperature zone was more concentrated and around 50 ℃ higher than circular planar burner. The v-shaped impinging structure in the rectangular outlet region enhanced the speed of chemical reaction by high temperature and made the unburned gas effectively blend in the central region. Furthermore, the computed results of non-intrusive diagnostics (PIV) revealed that the velocity and vorticity of v-shaped circular impinging burner were dramatically strengthened double compared to circular planar burner due to thermal buoyancy and shear stress. The mixing of unburned gas was proved by computing the horizontal velocity. There were also numerous vortices observed in the pictures of visualization, they swirled the outside air to the combustion field and increased the mixing of fuels and oxidants. According to the theoretical analyses and the experimental results discussed above, the increased flame interaction caused by the impinging structure takes several advantages to flame feature relative to circular planar burner. Based on the results of the first section, the v-shaped circular impinging burner was redesigned and discussed in the second section. The overall size was minified to reduce the interval between flames, and the angle of impinging was changed to find a suitable angle used in v-shaped circular impinging structure. The results show that the highest temperature was measured when the impinging angle was 60 degree. However, the highest magnitudes of velocity and vorticity were measured when the impinging angle was 30 degree. It seems possible that these results are due to different angles of impinging and chemical reaction. Therefore, what the angle of impinging should be chosen is dependent on the objective of use. This research may be the significant reference for the industrial and domestic applications using v-shaped circular impinging structure.